This invention relates to conduits such as those used in pneumatic conveying systems. Examples of such systems are fume exhaust systems, dust conveying systems, and ducting used in association with cotton gins. In particular, this invention relates to the coupling of conduit sections.
Conduits formed from sheet metal are widely used in pneumatic conveying systems. These conduits range in diameter from about 3 inches to about 80 inches (8-203 cm), and are formed by rolling sheet metal and joining the abutting edges by any of a number of conventional processes, some such as welding to produce a seamless product, and others such as rolling to produce a protruding seam. The formation of lock seams by the rolling is particularly efficient since it provides a strong reliable airtight seal in an efficient manner.
While lock seam tubing may be made with a straight seam following the axis of the tubing, a seam which spirals around the tubing has the advantage of permitting tubing of a wide range of diameters to be formed from a single width of sheet metal strip. Spiral lock seam tubing, however, presents a problem when sections of such tubing are joined end-to-end. In pneumatic conveying systems, the joints must be airtight to avoid loss of pressure inside the conduit and leakage to the surrounding atmosphere, particularly when superatmospheric pressures are present inside the conduit lines. The lock seam forms an irregularity in the surface contour of the conduit section which continues to the end edge. As a result, a simple circular clamping band will not provide a seal all the way around.
While the sealing problem may be approached by sealing on the conduit side opposite the side from which the lock seam protrudes, this is a disadvantage in certain conveying systems, such as those where material such as fibers is being conveyed. Such materials tend to catch on any protrusions on the inside surface of the conduit, thereby raising a risk of plugging inside the conduit or restricting the flow.
A further problem with internal couplings is the installation of such couplings when there is little clearance between the conduit sections. Internal couplings cannot be used when the space into which a conduit section is to be inserted lacks sufficient clearance in the lengthwise direction to permit insertion of the coupling end into the open end of the adjacent fitting or conduit.